This invention relates generally to a sootblower device for directing a fluid spray against heat exchanger surfaces in large-scale combustion devices, and particularly, to such a device for providing improvements in the uniformity of the cleaning effect provided.
Devices generally known as sootblowers have commonly performed cleaning surfaces within boilers, furnaces, or other devices in which a fossil fuel is combusted. Sootblowers typically employ water, steam, air, or a combination thereof, as a blowing medium, which is directed through one or more nozzles against encrustations of slag, ash, scale and/or other fouling materials, which become deposited on the surfaces.
Typical sootblowers of the long retracting type have a retractable lance tube which is periodically advanced into and withdrawn from the boiler and is simultaneously rotated such that one or more blowing medium nozzles at the end of the lance tube project blowing medium jets tracing helical paths.
Operators of large-scale boilers are continuously striving to improve the efficiency of their operation. The blowing medium discharge by sootblowers constitutes a thermal efficiency penalty for the overall operation of the boiler system. In addition, sootblowers further require substantial quantities of superheated steam or other pressurized fluid in order to effectively operate. Therefore, there is a desire to minimize the frequency of operation of sootblowers and the quantity of fluid which they discharge during each cleaning cycle.
Most efficient sootblower cleaning operation occurs when the jet of fluid emitted from the nozzle advances along the heat exchanger surfaces at a nearly uniform progression rate. Achieving such uniformity is difficult in situations where the distance between the sootblower nozzle and the surface being cleaned changes during the rotational motion of the lance tube. For example, if the lance tube is rotated as it is extended and retracted from the boiler and the surfaces being cleaned are planar surfaces such as pendant wall sections of water tubes, operating the lance tube at a constant rotational speed produces significant variations in the progression rate of the impact area of the cleaning medium stream advancing along a path on the surfaces. Thus, where the rate of jet progression is lowest, excessive quantities of sootblowing medium are used as compared with the amount required for effective cleaning. Moreover, physical deterioration of the heat exchanger surfaces may also occur where they are xe2x80x9cover cleanedxe2x80x9d in this manner. However, the cleaning requirements in areas where the jet progression rate is greatest may compel the operator to select rotation and translation speeds based on such xe2x80x9cworst casexe2x80x9d conditions, which further exacerbates the previously noted problems in the areas where jet progression is lowest.
Conventional sootblowers of the long retracting type use an elongated frame having a carriage assembly which is driven for movement along the frame. The lance tube is carried by the carriage. An internal drive mechanism causes a drive pinion gear to rotate which meshes with an elongated toothed rack fixed to the frame, driving the carriage for longitudinal motion. Through another set of gears, the lance tube is caused to rotate as the carriage and lance move longitudinally.
In order to overcome the previously noted disadvantages inherent in sootblower lance tubes operating at constant rotational speeds, designers of such systems have employed various solutions. One solution involves a complex drive system for the sootblower utilizing variable speed motor controllers coupled with position sensors which detect lance tube longitudinal and rotational position. Examples of such mechanisms are described in U.S. Pat. Nos. 5,337,438, 5,437,295, and Re. 32,517, which are commonly owned by the Assignee of this application and are hereby incorporated by reference. Although highly effective, the systems described by the previously referenced patents tend to impose a significant cost penalty due to the requirements of employing the previously noted controller and drive system elements. Thus, such prior art systems have cost disadvantages which may preclude their application where their capabilities may be effectively utilized. In addition to the previously noted shortcomings, such sophisticated sootblower systems pose maintenance challenges in the hostile environment in which they are employed.
One type of sootblower drive mechanism provides oscillating rotational motion. That is, the lance tube reversibly rotates through an arc and does not complete full rotations. Examples of such oscillating type sootblower systems are provided with reference to U.S. Pat. Nos. 4,177,539 and 4,351,082, both of which are commonly assigned with application and are hereby incorporated by reference. The Elting U.S. Pat. No. 4,177,539 disclose an oscillating mechanism using a so-called Scotch Yoke mechanism. This system produces an oscillating rotational motion for the lance tube, which could provide a varying angular speed. However, the mechanism required according to the Etling patent does not provide an adequate angular speed variation to prove constant jet progression and is a complex mechanism requiring specialized components and modifications to existing sootblower carriage systems.
Accordingly, there is a need in the art to provide a sootblower system which provides a more constant rate of jet progression without the disadvantages of sophisticated control systems as noted previously.
In accordance with the present invention, a lance tube drive system is disclosed which provides variable rotational speed, purely through the use of mechanical drive elements. In the described embodiment, a gear reduction unit driven through a power takeoff point of the carriage assembly is coupled through a meshing set of non-circular gears to provide a variable rotational speed output. This output is used to drive the lance tube for rotational motion. By establishing an indexed relative position between the lance tube nozzles and the non-circular gears, a desired variation in angular speed can be provided. Since it is purely mechanical, the system has inherent cost and reliability advantages over systems requiring sophisticated control components.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.